This invention relates to a method of positioning a wire electrode and a workpiece relative to each other in a wire cut electric discharge machine.
A conventional wire cut electric discharge machine, as shown in FIG. 2, comprises: a wire bobbin 1; a wire electrode 2 supplied from the wire bobbin 1; a brake roller 3; a powder brake 4 coupled directly to the brake roller 3; an auxiliary pulley 5; an upper pulley 6; a lower pulley 7; a take-up roller 8; an electric motor 9 for driving the take-up roller 8; a tachometer generator 10 for detecting the speed of the motor 9; a pinch roller 11 operating with the take-up roller 8 to clamp the wire electrode 2; a spring 12 for pushing the pinch roller 11 against the take- up roller; a workpiece 13; a surface plate 14 for fixing the workpiece 13; a table 15 for fixing and moving the surface plate 14; a ball screw 16 for moving the table 15; a nut 17 for converting rotation of the ball screw 16 into straight displacement of the table 15; an electric motor 18 for driving the ball screw 16; and an encoder 19 for detecting the angle of rotation of the motor 18.
The electric discharge machine further comprises: an upper wire guide member 20 for supporting the wire electrode 2 above the workpiece 13; a lower wire guide member 21 for supporting the wire electrode 2 below the workpiece 13; a current supplying element 22 which is brought into contact with the wire electrode 2 to supply electric current to it; a contact detector 23 for detecting the contact of the wire electrode 2 and the workpiece 13; and a numerical control unit 24 for supplying instructions to the brake 4, the motor 9 and the motor 18. The brake roller 3, the powder brake 4, and the auxiliary pulley 5 form a wire electrode tensioning unit 40b. The take-up roller 8, the motor 9, the tachometer generator 10, the pinch roller 11, and the spring 12 form a wire electrode running unit 50. The table 15, the ball screw 16, the nut 17, the motor 18, and the encoder 19 form a relative movement unit 60 for moving the wire electrode 2 and the workpiece 13 relative to each other.
The electric discharge machine thus organized operates as follows:
The wire electrode 2 is supplied from the wire bobbin 1, and is tensioned suitably according to its diameter being wound on the brake roller 3. In this operation, in order to prevent the slip of the wire electrode 2 thereby to tension it suitably, the wire electrode is wound one turn on the auxiliary pulley 5, and is then wound on the brake roller 3. The wire electrode 2 is laid over the upper pulley 6 and is positioned in place by means of the upper and lower wire guide members 20 and 21, and then laid over the lower pulley 7. The wire electrode thus laid is taken up while being clamped by the take-up roller 8 and the pinch roller 11. In this operation, the pinch roller 11 is pushed against the take-up roller 8 by the spring 12, so that the pinch roller 11 and the take-up roller 8 rotate without slip. Thus the wire electrode 2 has been laid under tension along the wire electrode running path.
It is necessary for the wire cut electric discharge machine needs to detect the relative position of the wire electrode 2 and the workpiece 13 before operation. This will be described in more detail.
The wire electrode 2 is positioned by adjusting its speed and tension. The motor 9 coupled to the take-up roller 8 is driven under the control of the numerical control unit 24 to run the wire electrode 2. The wire electrode 2 is run at a constant speed by a feedback signal from the tachometer generator 10.
The tension of the wire electrode 2 is adjusted by the numerical control unit 24 controlling the powder brake 4. The powder brake 4 produces a braking force in proportion to the current supplied thereto, and therefore the tension applied to the wire electrode 2 is proportional to the current supplied to the powder brake. The powder brake 4 works only when the wire electrode 2 is run. Therefore, the wire electrode 2 is positioned while being run at a predetermined speed and under a predetermined tension. When an instruction of rotation is applied to the motor 18 by the numerical control unit 24, the table 15 is moved slowly to the left until the wire electrode 2 is brought into contact with the workpiece 13. Thereupon, the contact detector 23 supplies a detection signal to the numerical control unit 24, and the position of the table 15 is detected from the output signal of the encoder 19 and stored in the numerical control unit 24. Thus, the relative position of the wire electrode 2 and the left end face of the workpiece 13 has been acknowledged.
The conventional method of positioning a wire electrode relative to a workpiece in a wire cut electric discharge machine is as described above. Therefore, the positioning operation cannot be achieved without running the wire electrode. When being run in this manner, the wire electrode is slightly oscillated, which decreases the accuracy and reproducibility in positioning the wire electrode relative to the workpiece.